In modern wireless systems, such as for example cellular wireless and fixed radio access wireless networks, there is a need for equipment operating with radio frequency signals, such as radio transceiver equipment in user equipment or at base stations or access points, which is economical to produce, while having high performance at radio frequencies. Increasingly high radio frequencies are being used as spectrum becomes scarce and demand for bandwidth increases. Furthermore, antenna systems are becoming increasingly sophisticated, often employing arrays of antenna elements to provide controlled beam shapes and/or MIMO (multiple input multiple output) transmission. Typically, radio frequency circuits are constructed with unbalanced transmission lines for transmission of radio frequency signals between components of the circuit, such as between amplifiers, filters, detectors, antennas and many other types of radio frequency component. An unbalanced transmission line comprises a signal conductor, such as a printed track of a circuit board, and a ground structure, such as a ground plane of a circuit board or a conductive ground plate, such as a milled aluminium plate. If one transmission line is connected to another transmission line, the signal conductor of one transmission line needs to be connected, at least at radio frequency, to the signal conductor of the other transmission line. It is also important that there is a good radio frequency connection between the ground structures of the respective transmission lines. This ensures a low loss connection between the transmission lines and, by ensuring that the conductors are referenced to the same ground voltage, reduces pick up of spurious signals.
Electronic equipment may be constructed with a ground plate having radio frequency electronic circuitry on both sides of it. For example, a ground plate may be a backing plate for an antenna array, and a radio transceiver may be mounted on the other side of the backing plate from the antenna array. There is typically a need to connect signals from one side of the ground plate to the other, for example between the antenna array and the transceiver.
Conventionally, signals may be connected through a ground plate using co-axial radio frequency connectors. Such co-axial connectors are typically made to tight mechanical tolerances and are relatively expensive, often being gold pated for example to ensure a good electrical connection and avoid corrosion. A poorly made or corroded connector may result in a poor radio frequency connection, resulting in signal loss or may even produce a connection with non-linear transmission characteristics resulting in generation of spurious signal components. There is a need for a low cost, high performance radio frequency connection, preferably tolerant of mechanical misalignment, between a transmission line on one side of a ground plate and a transmission line on the other side of the ground plate.
It is known to couple radio frequency signals between transmission lines on different layers of a printed circuit board using slot-coupled interconnects. The publication “Slot-Coupled Double-Sided Microstrip Interconnects and Couplers” by Ho, Fan and Chang, 1993 IEEE MTT-S Digest, discloses slot-coupled interconnects for coupling radio frequency signals between layers of a multi-layer printed circuit board, through an aperture in a ground plane forming a layer of the printed circuit board.
It is an object of the invention to mitigate the problems of the prior art.